This invention relates to a method of increasing the molecular weight of poly(alkylene carbonate) polyahls.
Poly(alkylene carbonate) polyahls are useful in preparing polyurethanes, polyureas and as surfactants.
Poly(alkylene carbonate) polyols can be prepared by the reaction of an alkylene carbonate such as ethylene carbonate with a glycol such as diethylene glycol.
For example, several workers have prepared poly(alkylene carbonate) polyols and related materials by controlling an equilibrium between the reaction materials of a diol and alkylene carbonate and the products of a poly(alkylene carbonate) polyol and monoethylene glycol. The reaction is controlled by the removal of monoethylene glycol.
Malkemus, U.S. Pat. No. 3,133,113 reacted ethylene carbonate and diethylene glycol at 125.degree. C. to 130.degree. C. under reduced pressure of 10 mm Hg in the presence of certain catalysts, e.g., mixed zinc borate-alkaline earth metal oxides, with concurrent removal of monoethylene glycol as distillate. This was followed by removal of starting material. This procedure is plagued by the presence of volatile ethylene carbonate which condenses as a solid throughout the system causing severe plugging and reducing ethylene carbonate conversion while monoethylene glycol is being removed. This process requires large excesses of ethylene carbonate.
Springmann et al., U.S. Pat. No. 3,313,782 further studied this process under reduced pressure in the presence of catalysts and set limits on the reaction conditions; the reaction temperatures must be lower than the boiling point of the alkylene carbonate, but high enough to distill off the monoethylene glycol formed.
Maximovich, U.S. Pat. No. 3,896,090 reacted ethylene carbonate with diethylene glycol and treated the reaction product under reduced pressure to remove the unreacted ethylene carbonate and diethylene glycol.
Lai et al., U.S. Pat. No. 4,131,731 used staged reductions in pressure during the reaction of alkylene carbonate with a diol, wherein the final stage was to remove monoethylene glycol. The patentees characterized their reaction conditions by stating that the alkylene carbonate must have a boiling point 4.9.degree. C. greater than monoethylene glycol. The chemistry based on the above equilibrium was improved by Buysch et al., U.S. Pat. No. 4,105,641 by carrying out the reactions in a solvent (e.g., cumene) capable of removing monoethylene glycol as an azeotrope with the solvent.
Stevens, in U.S. Pat. Nos. 3,248,414; 3,248,415 and 3,248,416, discloses the preparation of poly(alkylene carbonate) polyols from (1) carbon dioxide and 1,2-epoxides; (2) cyclic carbonates such as ethylene carbonate; or (3) from cyclic carbonates and a 1,2-epoxide. A minor amount of a polyol or a polyamine is employed as an initiator. The reaction is usually conducted in the presence of a metal carbonate, metal hydroxide, trisodium phosphate or a tertiary amine. In the Stevens' processes a poly(alkylene carbonate) polyol derived from ethylene carbonate and monoethylene glycol is exposed to temperatures of 160.degree. C. at 2 mm Hg pressure to remove unreacted ethylene carbonate.
Senet, U.S. Pat. No. 4,005,121 prepared poly(alkylene carbonate) polyols by reacting aliphatic diols with diethyl carbonate in a two-stage process. In the first stage, the reaction is carried out in the presence of a basic catalyst at a temperature below 140.degree. C. The basic catalyst is then destroyed and the reaction is completed in the presence of an acidic catalyst at a temperature of between 200.degree. C. and 250.degree. C. Reduced pressure is used toward the end of the second stage.
Poly(alkylene carbonate) polyols have also been prepared by polymerization of ethylene carbonates under pressure using basic catalysts and a minor amount of glycol as initiator, Buysch et al., U.S. Pat. No. 4,105,641. These products are low in carbonate and high in ether groups concentration due to decomposition of the ethylene carbonate.
Conventionally, the molecular weights of poly(alkylene carbonate) polyahls from alkylene carbonates have been controlled by either the stoichiometry of the reactants, that is, higher alkylene carbonate to initiator ratios for higher molecular weights, or the removal of monoethylene glycol from the reaction mixture wherein an ethylene carbonate to initiator equivalent ratio of about 1 is used. Catalysts are used in most cases, as reaction rates are very slow in the absence of a catalyst. When high alkylene carbonate to initiator ratios are used to make higher molecular weight poly(alkylene carbonate) polyahls, reaction rates drop severely as higher conversions are approached; long reaction times are required and the products are contaminated by unreacted alkylene carbonate. If temperatures are increased to increase the rate, the product decomposition occurs with CO.sub.2 loss. In the instant process, rates of molecular weight build are rapid without CO.sub.2 loss. The choice of catalyst has an effect on the molecular weight and the CO.sub.2 retention of the poly(alkylene carbonate) polyol. In each process the choice of the ratio of starting reactants and catalysts resulted in an upper limit on the molecular weight of the poly(alkylene carbonate) polyol which can be prepared. Furthermore, the products of such processes are of relatively low molecular weight and have a broad molecular weight range, that is, they have a high poly-dispersity index and are often contaminated with unreacted starting materials and relatively low molecular weight reaction intermediates. Moreover, the particular reactant ratio and catalyst used has a significant effect on the amount of carbon dioxide moieties in the backbone of the chain.
What is needed is a process for preparing higher molecular weight poly(alkylene carbonate) polyahls beyond the limitations imposed by the stoichiometry and catalyst used at reasonable reaction rates and free of low molecular weight contaminants. What is also needed is a process for making higher molecular weight poly(alkylene carbonate) polyahls with a relatively low poly-dispersity index. What is further needed is a process which allows the preparation of poly(alkylene carbonate) polyahls with higher carbon dioxide contents.